1. Field of the Invention
The invention relates to an improved process for preparing a uniform dispersion of titanium dioxide for forming a reflective layer for photographic paper.
2. Information Disclosure
Photographic base paper is used as an image-receiving base for prints produced by a number of different photographic processing systems, including chemical transfer offset, instant photography, and, in particular, the conventional negative-positive process system. The resulting print essentially consists of coated base paper and an image-containing layer which is adhered to the base. In the negative-positive process where the image-containing layer is referred to as the emulsion coating, a binder is usually employed under the emulsion coating to effect its adhesion to the base. Conventionally gelatin is used as the binder although alternative synthetic materials are used. A white pigment is generally incorporated in the gelatin. It is known that the sharpness of a photographic image depends on the extent of reflection of the impinging light off the white pigment. Therefore, it is an important object of all reflective binders to improve the reflection of the impinging light. This is achieved by employing white pigments with the highest indices of refraction, such as titanium dioxide, and by maintaining as high as possible a content of pigment in the gelatin. A very good dispersion generates a dense pigment packing in the support near the surface. Pigment agglomerates must not be generated in the gel, since they decrease the total light reflection, and they can result in disturbances and interferences during the casting of the coated support with light-sensitive emulsions.
In addition to titanium dioxide, the reflective binder layer, or "white pad" usually contains surfactants or dispersants, optical brighteners, and a very small amount of cyan dye to correct the whiteness of the TiO.sub.2. The formulation also includes an emulsion of polymer beads to provide improved surface texture, and may include antiseptics to retard the growth of microorganisms in the gel.
Conventional processes for the preparation of uniform fine particle dispersions of titanium dioxide in gelatin are time consuming and subject to periodic losses. The dispersions are made in three steps and all three steps must be executed with very tight tolerances. Dry titanium dioxide is wetted with water and two surfactants at a concentration of about 70%. The slurry is then run through a media mill and stored until the next step in the process is ready. In a second step in a large temperature-controlled vessel, the titanium dioxide slurry, distilled water, optical brighteners, polymeric bead emulsion, and a very small amount of a cyan dye are mixed for at least sixty minutes and heated to 40.degree. C. In a separate, large vessel a 12% solution of gelatin in water is prepared at 40.degree. C. The pH of the gelatin solution is matched to the pH of the titanium dioxide slurry and the two are mixed together. The mixing and rate of addition must be controlled properly to avoid the formation of foam and to avoid the formation of agglomerates. If agglomerates form, the dispersion must be filtered to remove them or if there are too many agglomerates, the dispersion must be discarded. Thus, there are several shortcomings with the process of the art: (1) three large temperature-control vessels are required, (2) if agglomerates are formed there are no corrective measures that can be taken to save the dispersion, and (3) the process is a three-step process.
There is thus a need for a single-step process which could be carried out in one vessel and which would avoid the problem of agglomerates in the final dispersion.
Known processes for making a reflective binder or "white pad" have employed a media mill to grind the solid component, TiO.sub.2, to produce fine particles, and then have combined the finely divided TiO.sub.2 with the liquid components to form the suspension for the white pad. It has now been surprisingly found that the entire formulation, containing both solids and liquid and including agglomerates, can be converted to a uniform dispersion by media milling. This is particularly unexpected because the agglomerates, which inevitably form when the ingredients are simply dumped together, contain not just titanium dioxide, which is known to be grindable, but also gelatin. Moreover, the volume of material passing through the media mill is greatly increased and the solids content is significantly diminished.